METAMORPHIC ROCKS. 15 
cienf to allow a very easy parting in thin plates. Under the microscope the*, rocks are 
found to have a surprisingly large amount of feldspar, in consideration of their very dark 
color. They are typical "greenstone schists." -The feldspar oeew in small, rudely 
polygonal grains, in' many specimens built up zonally, commonly untwi.t»"'d, and pe needy 
clear. It belongs mainly to the variety albite, with a less amount of oligcgi -e. Epidote 
is often found in abundance, both in irregular interstitial grains and as lath-shaped prisms 
poikilitically penetrated by other constituents of the rock. Zoisite occurs in a p > Her 
similar to epidote and is on. the whole more common. These two minerals m ■ c^k 
together and in many cases can be distinguished only in polarized light. Calcite likewise 
pccurs in numerous specimens, in irregular grains and in lath-shaped individuals, and 
was undoubtedly* present when the recrystallization of the rock took place. Calcite is 
usually not abundant in those specimens which contain much of • the calcium silicates 
epidote and zoisite, or, if it is, it occurs principally in bands or zones in which they are 
sparingly present. Quartz, either formed by the removal of bases from some of the sili- 
cates or introduced by the waters producing metamorphism, perhaps also in part primary, 
forms mosaics of small clear grains which are distinguishable from albite only in con- 
vergent polarized light. 
The dark constituent is the typical amphibole of the " greenstone schists" and is proba- 
bly uralite. It occurs in well-formed prisms and in irregular, elongated grains, but in no 
case was it seen developed as a pseudomorph after pyroxene. As a rule, it is rather strongly 
pleochroic in bluish and brownish greens and has a not very strong double refraction. 
Black iron ore is generally abundant and is associated particularly with the amphibole. 
The most common occurrence is in irregular grains, but in some cases it is present as crys- 
tals of definite outline, usually too small to 'be accurately determined as to form, but some- 
times showing a pair of predominant parallel faces that give the crystal a flattened appear- 
ance and suggests ilmenite. Many of the grains are surrounded by borders of sphene, and 
in some cases the change to this mineral has been complete. In a few instances a mineral 
which is probably rutile accompanies the titanite. The iron ore is therefore titaniferoiis 
in most or all eases. Small "broken grains of zircon are present in a few specimens. 
In those specimens which contain considerable calcite, epidote, or zoisite it is not uncom- 
mon to find irregular grains, turbid and decomposed but still showing the twin lines, of a 
moderately calcic plagioclase, usually labradorite. The rocks free from calcite, epidote, 
and zoisite have in general a cleaner, better-defined appearance than those which are not. 
It appears probable that the crystallization of these secondary lime-bearing minerals is 
only an intermediate step in (he complete metamorphism of the original rock, the ultimate 
result being their solution and removal, leaving a rock decidedly less rich in lime than the 
original. 
These amphibolites thus present a typical example of the original metamorphism of a 
rather basic igneous rock, probably having the composition of a gabbro, with the production 
of a wholly recrystallized and altered schist. 
Rocks of this character are extremely common in the western part of the area examined 
and probably are the most widely developed variety in that part. In central York County, 
as has been said, the degree of metamorphism which they have suffered is not so great. 
There they possess a more granitic texture and pass gradually into gabbros. In the 
eastern part of the area amphibolites were less commonly observed, although a study of 
the areal geology might reveal them in abundance. 
QUARTZ-SERICITE SCHIST. 
Dense, siliceous rocks, usually of rather light gray color, more or less schistose, occur 
in several portions of the area and may have a considerable extent. A peculiar silvery, 
mottled luster, frequently seen on the planes of schistosity, has won for these rocks the 
very suggestive name applied by jjie miners, "fislnscale slate." The appropriateness of 
this name is especially apparent when the rocks are wet and viewed by the flickering light 
of tile miner's candle. 
